rebase -i: Use "test -n" instead of "test ! -z"

It is a tiny bit simpler.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -156,7 +156,7 @@ pick_one () {
 	output git rev-parse --verify $sha1 || die "Invalid commit name: $sha1"
 	test -d "$REWRITTEN" &&
 		pick_one_preserving_merges "$@" && return
-	if test ! -z "$REBASE_ROOT"
+	if test -n "$REBASE_ROOT"
 	then
 		output git cherry-pick "$@"
 		return